1. Field of the Invention
This invention relates to an optical information recording-reproducing apparatus for recording or reproducing information on or from an optical information recording medium such as an optical disc, and particularly to an offset adjusting device for automatically adjusting a focusing offset.
2. Related Background Art
FIG. 1 of the accompanying drawings is a block diagram showing an example of the signal reproducing system of an optical disc apparatus. In FIG. 1, the reference numeral 1 designates an auto gain control circuit (hereinafter abbreviated as the AGC circuit) for controlling the amplitude level of a reproduction RF signal to a constant level. The reproduction RF signal is a reproduction signal read out from a recording medium (not shown) such as an optical disc, and its amplitude level Fluctuates because of various factors, such as the irregularities of the intensity of a beam of reproducing light, the transmittance of an optical system, the reflectance of the optical disc, etc., or a stain on the optical system and on the optical disc. Such fluctuation of the reproduction RF signal causes a binarization error during binarization, and this becomes the cause of erroneously reproduced data. So, in the AGC circuit 1, the amplitude level of the reproduction RF signal is controlled to be at a constant level and a reproduction RF signal of a constant level is supplied to a binarizing circuit 2, whereby the binarization error is prevented and the reproduction of data is effected accurately.
The AGC circuit 1 is comprised of a variable gain amplifier (hereinafter abbreviated as the VGA) 3, a full wave rectifier 4 and a comparator 5, and to control the reproduction RF signal, the output signal of the VGA 3 is first full-wave-rectified by the full wave rectifier 4 and is converted into a DC voltage. The thus obtained DC voltage is compared with a reference value by the comparator 5, and the gain of the VGA 3 is varied so that the difference between the DC voltage and the reference value may become zero, whereby the output signal level of the VGA 3 is controlled to a constant level. Thus, feedback is applied to the gain of the VGA 3, and even if the amplitude level of the reproduction RF signal Fluctuates from various factors, there can be obtained a reproduction RF signal of a constant level. This reproduction RF signal is binarized at a predetermined slice level by the binarizing circuit 2, whereafter it is subjected to signal processing such as synchronization and demodulation process by a signal processing circuit, not shown, and reproduction data is produced.
On the other hand, during the starting of the apparatus or during the interchange of the optical disc, the adjustment of a focus offset is effected by the use of the reproduction RF signal. During the adjustment of this offset, a microcomputer 8 sets predetermined offset in a D/A converter 9, and applies the offset to the servo loop of a focusing servo circuit 10. At this time, the reproduction RF signal is full-wave-rectified by a full wave rectifier 6, and thereafter is digitalized by an A/D converter 7 and is introduced into the microcomputer 8. The microcomputer 8 stepwisely changes the offset and measures the amplitude level of the reproduction RF signal each time, and detects the offset when the amplitude level becomes a maximum. That is, the microcomputer searches for the offset during the best focusing when the amplitude of the reproduction RF signal becomes a maximum, and sets it in the D/A converter 9, whereby offset adjustment is effected.
That is, in focus control for forming a minute focus on the optical disc, if the focus deviates (offset occurs), the light spot will become large and a minute signal on the disc will become unreproducible. Also, the focus position differs delicately depending on the thickness of the substrate of the disc and the material of which the substrate of the disc is composed. In addition, the focus can deviate from the ideal focus due to a variation in the optical system with time and therefore, the offset adjustment of the focusing servo circuit is affected so that the best focus may be provided when the optical disc is interchanged or the apparatus is started, as previously mentioned.
Now, in the prior art, as shown in FIG. 1, the full wave rectifier 4 is provided in the AGC circuit 1 and the full wave rectifier 6 is also provided for the adjustment of the focusing offset and therefore, it would occur to one to make these rectifiers common and simplify the circuit construction. However, when the output signal of the full wave rectifier 4 is used for the adjustment of the offset of the focusing servo circuit, the reproduction RF signal of the output of the VGA 3 assumes a constant level due to the control by the AGC circuit 1, and no change will appear in the output of the full wave rectifier 4. That is, if in order to search for the best focus position, a focusing offset is gradually added to the focus position to thereby shift the focus from the defocus state to the best focus state, the reproduction RF signal ought to gradually increase, but since the control of the AGC circuit 1 is performed, no change appears in the output of the full wave rectifier 4. Therefore, it has been difficult to use a single full wave rectifier and two full wave rectifiers have been necessary for the adjustment of focusing servo circuit and for the control of the level of the reproduction RF signal, respectively.
Also, when the output of the full wave rectifier 6 is introduced to the A/D converter 7 to thereby adjust the focusing offset, the reproduction RF signal Fluctuates because of various factors before AGC is applied to it and therefore, when the amplitude level of the reproduction RF signal is small, the variation in the input signal to the A/D converter 7 will not appear as a variation in the number of the bits thereof. This has led to a problem that in such a case, the amplitude of the reproduction RF signal cannot be accurately measured and the accuracy of the adjustment of the offset of focusing servo becomes poor.